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Posted at 9:30 PM ET, 09/22/2010

Storm threat wanes, muggy overnight

By Ian Livingston

Day 64 of 90+ today; More heat tomorrow and Friday

* Global warming or what? | When weather was a secret | NatCast *
* Outside now? Radar, lightning, temps & more: Weather Wall *

9:30 p.m. update: The Severe Thunderstorm Watch has been discontinued for the region. Expect warm and humid conditions for the rest of the evening.

Through Tonight (from earlier): Isolated showers or thunderstorms, some potentially severe with strong winds and/or hail early, may continue into the evening. As the sun sets, they will dwindle leaving us mostly clear overnight. Lows fall to the mid-60s in the suburbs to the lower 70s downtown.

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Tomorrow (Thursday): Mostly sunny skies greet the first day of autumn, but it won't feel too much like fall. We're set up for another hot and somewhat muggy one as highs reach near 90 or into the lower 90s.

See Dan Stillman's forecast through the weekend. And if you haven't already, join us on Facebook and follow us on Twitter.

7:40 p.m. update: A new Severe Thunderstorm Warning is up, mainly for northern P.G. County and east into central to northern Anne Arundel. This storm is exiting southeastern Montgomery County and moving east, and should impact the warned areas during the next 45 minutes or so. Winds near to over 60 mph are possible.

6:30 p.m. update: Though some isolated showers/storms linger throughout the region, the severe thunderstorm watch has been canceled. [Correction (7:50 pm): Severe T'storm Watch only cancelled for southern portions of viewing area. Still in effect for immediate metro area and points north.]

4:55 p.m. update: Isolated severe weather that popped up this afternoon has largely dwindled back into a few showers with perhaps a remnant rumble or two. Many places have not seen any rain and probably won't for the rest of the evening. Overall, it looks like activity will be minimal from here on into the evening, and we'll lose the leftover stormy ingredients as the sun goes down. Outside a few hail reports in Loudoun County, the main impact of these storms was brief heavy rain and some lightning.

4:10 p.m. update: Showers and storms continue to impact the area, but they have recently lost a bit of punch as they head toward the city. This may be a temporary downward pulse, as heat and humidity are still quite prevalent as fuel sources. Reports of hail up to about golf ball size have been come in from places like Leesburg in Loudoun County. Showers and storms should impact the immediate D.C. area over the next 30-60 minutes before heading east of the Beltway, though some additional activity has popped up behind and may linger.

3:30 p.m. update: Thunderstorms continue to develop and move eastward across the area, and while they are isolated to scattered, the main batch appears to be targeting the central part of the metro area. The severe risk with these storms is brief gusty winds as high as 60 mph along with small to medium sized hail. Reports of hail up to golf ball size have come in from Leesburg. The showers and storms should progress toward D.C. in the next 60-90 minutes.

2:30 p.m. update: Astronomical summer is ending on a summery note with high heat and the threat of severe storms. One such storm has popped up to the west and is now moving through northern Loudoun County with the risk of damaging winds up to 60 mph and small hail. So far, this is the main game around here, and it's covering a small area. Many spots may end up not being impacted, but additional storm formation is possible and a Severe Thunderstorm Watch has been issued until 10 p.m.

By Ian Livingston  | September 22, 2010; 9:30 PM ET
Categories:  Forecasts  
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Next: NatCast: A summery afternoon!


Some intense lightning strikes and small (larger than pea but smaller than dime)sized hail in the Purcellville/Hamilton area. At least it's raining, though!

Posted by: technomuse | September 22, 2010 3:02 PM | Report abuse

What's the timeline for the t'storms to hit downtown DC? Or are they headed this way for sure? thanx

Posted by: JerryFloyd1 | September 22, 2010 3:33 PM | Report abuse

Wicked lightning, thunder and pouring rain in Ashburn!

Posted by: natsncats | September 22, 2010 3:40 PM | Report abuse

At last some rain.
Of course it has to come at rush hour.

Posted by: FIREDRAGON47 | September 22, 2010 3:45 PM | Report abuse

Still mostly sunny in NW dc.

Posted by: BruinGirl2001 | September 22, 2010 4:11 PM | Report abuse


Over the next 45 min. or so for downtown D.C., though as Ian mentions above, the showers and storms seem to be losing their punch as they approach the city.

Posted by: Dan-CapitalWeatherGang | September 22, 2010 4:18 PM | Report abuse

No rain here in Spoty. Holding steady at .13" this month, & only 5.5" since June 3.

Posted by: VaTechBob | September 22, 2010 4:27 PM | Report abuse

Rain fell apart but the temp keeps going up. 94 now at DC!

Posted by: Ian-CapitalWeatherGang | September 22, 2010 5:05 PM | Report abuse

It rained approximately 4 1/2 minutes here and then full blown sunshine. It feels disgusting outside. What happened to my beautiful and comfortable Fall weather?!

Posted by: HokieTerp | September 22, 2010 5:32 PM | Report abuse

Ho hum. Missed another one in NW Montgomery.

Posted by: MKadyman | September 22, 2010 5:32 PM | Report abuse

Oops. "Here" is Herndon!

Posted by: HokieTerp | September 22, 2010 5:36 PM | Report abuse

Thanks, Ian, and you're right the rain fizzled. I busted it to get home before the rain and not enough precip fell in Glover Park to wet the sidewalk.

And with a high of 95 today, we're up to 1,994 cooling degree days, only 12 short of 1980's 2,006. That record will surely fall tomorrow or Friday.

Posted by: JerryFloyd1 | September 22, 2010 5:40 PM | Report abuse

Sorry, about should have read, "You're right Dan..."

Posted by: JerryFloyd1 | September 22, 2010 5:42 PM | Report abuse

Just wanted to mention that on the first day of astronomical fall in 1980, the DCA high was 101 degrees, a figure Ian verified this summer. So hot though it was, today's 95 was kinda' wimpy.

I'm betting we reach 66 90 days on Friday and that's mercifully the end of it for 2010.

Posted by: JerryFloyd1 | September 22, 2010 5:55 PM | Report abuse

Jerry, actually I think you have something mixed up on that stat. The last 100 I see (101) in 1980 was Sept 2. Interestingly enough the high on the 22nd of Sept 1980 was 95 as well, but the low was a bit warmer then -- 73 instead of 67 today. The final 90 of 1980 was the 23rd of Sept. I'm still looking it over, but it appears that 1980 does still edge out this year for astronomical summer when comparing the average temperature. But in the end they seem pretty similar... and of course this years heat came on harder earlier.

Posted by: Ian-CapitalWeatherGang | September 22, 2010 6:03 PM | Report abuse

...Now it's College Park's turn to rain!

Posted by: kolya02 | September 22, 2010 6:18 PM | Report abuse

Leesburg had some crazy pea and grape-sized hail... I mean it was insane! Also had steady downpours around 5ish but it finally appears to be drying up out there.

Hadn't had a good storm in awhile - a little excitement on this pleasantly warm day. :)

Posted by: rumbly45 | September 22, 2010 6:26 PM | Report abuse

Thanks for clarifying, Ian. Was the 101 on Sept 2, 1980 the latest 100-degree-plus reading? Maybe that's where I got mixed up.

As for which year was hotter, this year or 1980, 2010 will win two out of three indicators, i.e., meteorological year average temp and tomorrow or Friday, cooling degree days. So, it's fair to say that heatwise 2010 edges out 1980. And when it comes to snow, 2010 wins by a whiteout.

Given how quickly snowmageddon and snowpocalypse became linga franca (although they are sometimes confused on the intranet), the media are apt to be eagerly awaiting CWG's moniker for next big snowstorm. ; ))

Posted by: JerryFloyd1 | September 22, 2010 7:11 PM | Report abuse

HOLY MOLY!! Huge thunder here in bethesda!

Posted by: samdman95 | September 22, 2010 7:23 PM | Report abuse

Rumbles of thunder approaching southern Silver Spring.

Posted by: Murre | September 22, 2010 7:27 PM | Report abuse

OK, what the heck just happened in College Park? And please, is it done yet?

Once will do for that much thunder and lightning, thank you very much.

Now off to calm down the dog.

Posted by: cady123 | September 22, 2010 8:37 PM | Report abuse

Jerry, if you have a site for the cooling day info and can post a link here tonight that would be great. Thanks in advance if you see this.

Posted by: Ian-CapitalWeatherGang | September 22, 2010 9:18 PM | Report abuse

Amazing lightning strikes as the storm bypassed the District in its entirety to the north/east. Sure hope you were lucky enough to have some rain in College Park, cady123, because we did not have a single drop down the road in Michigan Park.

But I'll continue "wishin', and hopin', and prayin'" for RAIN!!!

Posted by: TominMichiganParkDC | September 22, 2010 9:40 PM | Report abuse

What are cooling degree days? I did some (by no means diligent) internet research and couldn't find a decent explanation- I found something about cooling DEGREES and about somehow trading futures about temperatures, but nothing about the days themselves. Since the experts are here on the CWG blog, I hope someone can help.

Posted by: kolya02 | September 22, 2010 9:43 PM | Report abuse

hi kolya02 - How much A/C use is needed X how long a duration? That is cooling degree days. You can think of it as a measure of how hot a location was over a period of time, relative to a base temperature. Usually the base temperature is 65 degrees. The number of cooling degree-days for a single day would be the difference between that day's average temperature the base temp of 65F.

Add one degree-day for each degree above the average daily temp is, compared to 65F. Degree-days = zero if the daily average temperature is less than or equal to 65F. The number of cooling degree-days for a longer period of time is the sum of the daily cooling degree-days for the days in that period. Annual cooling degree-days averaged over many years are called "Normal" Cooling Degree-Days.

Average daily temperature is the mean of the maximum and minimum temperatures for a 24-hour period. Cooling degree-days can also be calculated using a base temperature other than 65 degrees. The computation is performed in an analogous manner.

Example: Chinatown (DC) today got as low as 67F, as high as 93F. To get avg temperature of the day: 93+67=160; 160/2 (readings) gives us=80F. Then, 80(today's avg temp)-65(mean)=15 degree-days today. In a broad sense, it means today was very tempting to use air conditioning! :)

Posted by: Camden-CapitalWeatherGang | September 22, 2010 10:08 PM | Report abuse

So close to that record of 67.

Posted by: crazer | September 22, 2010 10:23 PM | Report abuse

The rain missed my area, though I saw a nice rainbow just east of Ballston.

Now back to 79 at 11:30 PM.

Posted by: Bombo47jea | September 22, 2010 11:34 PM | Report abuse

Thanks for the very descriptive explanation, Camden! I'm curious who chose 65 degrees as a baseline and why; we all know a "comfortable" temperature is highly subjective and also relative. Many people in the winter will set their thermostats to ixty-five to seventy to be comfortable, yet cooling degree day rules dictate that those same people in the summer would be turning on their ACs at the sixty-five to seventy degree range. Interesting.

Posted by: kolya02 | September 22, 2010 11:53 PM | Report abuse

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